Polysilicon processing using an anti-reflective dual layer hardmask for 193 nm lithography

ABSTRACT

A lithographic method of forming submicron polysilicon features on a semiconductor substrate, including the steps of coating said substrate with an anti-reflective coating (ARC) comprising two layers having matched indices of refraction (n) and extinction coefficient (k) selected to reduce reflection to less than 1% with 193 nm wavelength exposure. The ARC is subsequently patterned to serve as an etch hardmask. Preferably the ARC mask consists of a first layer of between 300 and 1500 angstroms of silicon rich silicon nitride having an extinction coefficient of from 0.77 to 1.07, and a second layer of between 170 and 320 angstroms of silicon oxynitride having an extinction coefficient of about 0.32.

FILED OF THE INVENTION

[0001] The invention relates generally to the manufacture ofsemiconductor devices, and more specifically to the method for definingprecise, narrow polysilicon features.

BACKGROUND OF THE INVENTION

[0002] The semiconductor industries continuing drive toward integratedcircuits whose geometric features are decreasing has in turn led to theneed for photolithographic techniques using shorter wavelengths in themid and deep ultraviolet (DUV) spectrum to achieve fine features. In theprocess of defining very fine patterns, optical effects are oftenexperienced which lead to distortion of images in the photoresist thatare directly responsible for line width variations, and which in turncan compromise device performance.

[0003] Many of the optical effects can be attributed to reflectivity ofthe underlying layers of materials, such as polysilicon and metals,which can produce spatial variations in the radiation intensity in thephotoresist, and in turn result in non-uniform line width development.Radiation can also scatter from the substrate and photoresist interfacesinto areas where exposure is not intended, again resulting in line widthvariation.

[0004] As the wavelength of exposure sources is shortened to bringimproved resolution by minimizing diffraction limitations, thedifficulty in controlling reflections is increased. In an attempt tocircumvent the reflection problems, a number of antireflective coatings(ARC) to be interposed between the substrate and photoresist have beendeveloped, largely for specific applications, and with varyingshortcomings.

[0005] To further complicate the problem, photoresists for shortwavelength exposure sources to deep ultraviolet (DUV) light arenecessarily very thin, and either do not withstand, or are undercutduring the etch process resulting in further deterioration of the lineresolution. Clean-up and removal of both the resist, and theantireflective coating can present additional problems in themanufacturing process of sub-micron features.

[0006] As lithography moves to the 193 nm (nanometer) wavelength of anArF excimer laser light, a need exists for a method to form sub-micronintegrated circuit patterns which overlay varying topography, and oftenhighly reflective substrate materials. In particular, defining precise,sub-micron features in relatively thick doped and undoped polysiliconover gate oxide presents a significant challenge to the industry. Aninorganic antireflective coating of silicon oxynitride (SixOyNz) hasbeen used in the industry, and while it has advantages, its selectivityto oxide, and slow removal rate with phosphoric acid post etch clean-uphas an adverse effect on the polysilicon line definition, and may resultin damage to active areas. Alternately, a bilayer of silicon oxynitrideover doped silicon oxide has been proposed. However, the opticalproperties of the oxide have a narrow process window, an undesirablefeature for volume manufacturing, and further the process is complicatedby the requirement for a special tool for removal.

[0007] Therefore, an anti-reflective coating for deep uv exposure in the193 nm wavelength region which is compatible with polysilicon etch andclean-up processes, and which supports volume manufacturing requirementsof sub-micron polysilicon features is clearly needed by the industry.

SUMMARY OF THE INVENTION

[0008] It is an objective of the present invention to provide a methodfor accurately defining sub-micron polysilicon features on asemiconductor device using 193 nm wavelength lithography.

[0009] It is further an object of this invention to provide an duallayer coating which serves both as an in situ etch mask, and as a lowreflectivity coating for deep UV exposure.

[0010] It is an object of the invention to provide an inorganic coatinghaving reflectivity of less than 1% at 193 nm wavelength exposure.

[0011] It is an object of the invention to provide a method for improvedlithographic depth of focus as a result of compatibility with thinphotoresist and antireflective properties of the ARC.

[0012] It is an object of this invention to provide a low reflectivityhard mask for polysilicon processing having a large film thicknesswindow.

[0013] It is an object of the invention to provide a coating comprisingmultiple layers wherein the thickness, the extinction coefficient andindex of refraction are matched to predict, and minimize reflectivity.

[0014] It is an object of this invention to provide a hard mask which iscompatible with phosphoric acid post polysilicon etch clean-up withoutdeterioration of underlying oxide and/or active areas.

[0015] These and other objectives will be met by sandwiching between thepolysilicon and photoresist layers, an ARC (anti-reflective coating)bilayer wherein the materials have matched index of refraction (n) andextinction coefficient (k) specifically to minimize reflection to lessthan 1% with 193 nm wavelength exposure, and which is subsequentlypatterned to serve as an etch hard mask. Preferably the ARC maskconsists of a bottom layer of greater than 300 angstroms, and less than1500 angstroms of silicon rich silicon nitride having an extinctioncoefficient of from 0.77 to 1.07, and a top layer of about 250 angstromsof silicon oxynitride having an extinction coefficient of about 0.32.The silicon nitride is in direct contact with a polysilicon layeroverlying a gate oxide, or other dielectric layer. An etch hard mask isformed from the ARC bilayer by etching in selected areas unprotected byphotoresist. The resist is removed by plasma ashing, and the exposedpolysilicon etched along with the silicon oxynitride layer, leaving onlythe silicon nitride to be removed by a phosphoric acid post polysiliconetch clean-up, which does not damage active moat and gate areas.

[0016] These and other features and advantages of the present inventionwill become apparent from the following description which is to be readin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 provides a cross sectional view showing the layers ofmaterial for fabricating a semiconductor device using the inorganic ARCand hard mask of this invention.

[0018]FIG. 2 is a cross-sectional view of the semiconductor device ofFIG. 1 showing the resist layer patterned in accordance with the presentinvention.

[0019]FIG. 3 is a cross-sectional view of the semiconductor device ofFIG. 1 showing the photoresit mask and patterned ARC in accordance withthe present invention.

[0020]FIG. 4 is cross-sectional view of the semiconductor device of FIG.1 with the hard mask of the current invention.

[0021]FIG. 5a is a cross-sectional view of the semiconductor device ofFIG. 1 with the patterned silicon rich silicon nitride and polysilicon.

[0022]FIG. 5b is a cross-sectional view of the semiconductor device ofFIG. 1 with the completed polysilicon feature.

DETAILED DESCRIPTION OF THE DRAWINGS

[0023] According to the invention, a method is provided for fabricatinga semiconductor device having narrow, sharply defined polysiliconfeatures by using deep UV exposure, such as 193 nanometers (nm). Theinvention includes a patterned bilayer of inorganic materials sandwichedbetween the polysilicon and photoresist layers, serving both as anantireflective coating having highly selective optical properties, andas a hard mask, stable during the etch and clean-up processes.

[0024]FIG. 1 is a cross-section of the layers involved in processingpolysilicon structures of a semiconductor device using the inorganicantireflective coating (ARC), and hard mask of the invention.

[0025] A very thin oxide or other dielectric layer 101 exists betweenthe provided silicon substrate 100, and a deposited polysilicon layer102 in the range of about 1200 to 2500 angstroms (A) thickness.

[0026] A dual antireflective thin film layer of materials, in thepreferred embodiment includes silicon oxynitride 104 and silicon richsilicon nitride 103 deposited between the polysilicon 102, and a thinlayer of photoresist 105. The photoresist, in the range of 2000 to 3000angstroms in thickness is preferably a positive acting deep UV resist,such as PAR 707 or 710 from Sumitomo Chemicals.

[0027] A unique feature of the invention is the inorganic bilayer havingspecific antireflective properties which improve depth of focus of thelithographic process, have a large process window, and form a hard maskwhich is able to withstand the etch process without deterioration ofeither the polysilicon line width, or the underlying oxide, moat, orother active areas.

[0028] Silicon oxynitride (SixOyNz) has been demonstrated as anantireflective coating for deep UV resist exposures 1 a largely becauseof the low index of refraction or “n” value. Such films have beenmanufactured having index of refraction in the range of 1.8 to 1.9, andhaving extinction coefficients or “k” values which can be varied from0.32 to 0.86. However, the removal of these materials is difficultwithout resulting in damage to the moat and the gate line width, thusmaking the single SixOyNz film unsatisfactory for manufacturingsemiconductor devices.

[0029] Alternately, silicon rich silicon nitride (SixNy) films have beenused as antireflective coatings and/or as hard masks, however theiroptical constants in the 193 nm wavelength range are not acceptable.Films having high “k” values, in the range of 0.7 to 1.1, and low “n”values in the range of 2.1 to 2.3 have high reflectivity. On the otherhand, SixNy films having low “k” values, in the range of 0.2 to 0.5, andhigh “n” values in the range of 2.35 to 2.45 provide low reflectivity at500-550 angstroms thickness, but the film thickness process window isvery small, and the film is not thick enough to permit etching thickpolysilicon of about 2200 to 2500 A thickness.

[0030] Simulations show that in order to provide a coating with lessthan 1% reflection, the ARC in contact with the resist must have ratherprecise “n” and “k” values, and that such values can be achieved bymatching the properties of a bilayer of materials.

[0031] A film having a low extinction coefficient of about 0.3, for theupper layer, or that layer in contact with the photoresist, and athickness of about 250 angstroms allows use of a material having a largeextinction coefficient of about 1 for the bottom layer. The reflectivityis largely independent of the thickness of the lower layer, providedthat it is greater than 300 angstroms, and the “k” value is in the rangeof 0.77 to 1.07. Thickness the lower layer is allowed between 300 and1500 angstroms in order to be compatible with gate pattern and etchprocesses.

[0032] A second or upper layer of SixOyNz 104 in the range of 170 to 230angstroms with an extinction coefficient of 0.32 paired with a first, orbottom layer of SixNy 103, greater than 300 angstroms thick, and havingan extinction coefficient of about 1.02 was shown to provide ananti-reflective coating having less than 1% reflectivity. Thiscombination of non-stiochiometric plasma deposited films, SixOyNz 104and SixNy 103, is compatible with deposition and etching processes, andwith lithographic requirements for 193 nm exposure, and therefore is thepreferred embodiment of the hard mask ARC for defining submicronpolysilicon features.

[0033] Alternate material selections having the necessary “n” and “k”values are acceptable as ARC coatings for 193 nm wavelength exposure,provided that the masking functions and removal process are compatiblewith manufacturing technology.

[0034]FIGS. 1 through 5a and 5 b illustrate, in cross section, processsteps in accordance with an embodiment of the invention whereinsubmicron polysilicon structures of an integrated circuit device arefabricated using 193 nm wavelength exposure. FIG. 1 illustrates theunpatterned or etched multiple layers of the structure including, apolysilicon layer 102 in the range of 1600 to 2500 angstroms thicknessoverlying a thin silicon oxide film. These layers are deposited or grownby known manufacturing processes.

[0035] The anti-reflective coating films 103 and 104 are deposited in aparallel plate PECVD (plasma enhanced chemical vapor deposition)reactor, such as a Centura Mainframe, DxZ process chamber as supplied byApplied Materials. The deposition processes for the bilayer ARCmaterials 103/104 using the exemplary reactor includes a processtemperature of 350 deg. C., pressure of 6.2 Torr, and an RF power of 130Watts for SixNy 103, and RF power of 120 Watts for SixOyNz 104. For thesilicon rich silicon nitride 103 deposition, SiH₄ is introduced at 88sccm, NH₃ at 225 sccm and He at 1900 sccm. Following the silicon nitride103 deposition, a silicon oxynitride 104 is formed in the same chamberby introducing SiH₄ at 63 sccm, N₂ 0 at 187 sccm, and He at 1900 sccm.The latter film 104 incorporates a high concentration of hydrogen, inthe range of 20-30 atomic percent, in the film. The preferred depositionprocess parameters, developed to provide the required “n” and “k” valuesmay vary with the specific chamber and reactor design.

[0036] A photoresist 105, preferably a positive acting resist, such asPAR-707 or PAR-710 as supplied by Sumitomo Chemicals is deposited atopthe antireflective SixOyNz layer using conventional spin coatingtechniques. The very thin photoresist 105, of approximately 2100 to 3000angstroms thickness, is kept thin in order to improve depth of focus forthe deep UV exposure, as well as to allow easy of resist removal.

[0037] In accordance with the invention unwanted reflection of radiationfrom underlying polysilicon during lithographic processes is alleviatedby use of the specific combined anti-reflective properties of films ofSixOyNz 103 and SixNy 104. Simulations were used to predict theantireflective properties, and materials experimentally synthesized toverify the predictions.

[0038] In FIG. 2, the photoresist 205, is lithographically patterned inselected areas using 193 nm wavelength exposure 210 from for example, anArF excimer laser source. After developing, the resulting well definedphotoresist pattern 205, as shown in FIG. 2, forms a resist mask whichoutlines features subsequently to be etched into an inorganic hard maskof SixOyNz 304 and SixNy 303, as shown in FIG. 3.

[0039] Etch and overetch of the hard mask layers is accomplished in acommercially available plasma etch reactor using CF₄ and O₂.

[0040] Following etch of the SixOyNz 304 and SixNy 303, the photoresist305 is removed by an oxygen ash step, which may be accomplished in thesame reactor. In the exemplary process, the photoresist is rapidlyremoved by an ash process using an O₂ flow rate of 100 sccm and N₂ flowof 200 sccm at 10 mTorr. The resulting hard mask 403/404 structure isillustrated in FIG. 4.

[0041] Using the inorganic hard mask, well defined sub-micronpolysilicon structures are formed by plasma etching the unprotectedareas. The SixOyNz 404 portion of the hard mask is removed along withthe unprotected polysilicon during the etch process, leaving only theSixNy as the final mask. The resulting structure including thepolysilicon structure 502 and SixNy 503 is illustrated in FIG. 5a.Polysilicon etch is accomplished in a commercially available plasma etchequipment using an etchant such as CF₄. The specific etch processparameters are equipment, and polysilicon thickness dependent.

[0042] Fabrication of the polysilicon feature is completed by removingthe silicon rich silicon nitride 503 using conventional hot phosphoricacid post polysilicon etch clean-up processing. The completedpolysilicon feature 502 is illustrated in FIG. 5b. The rapid removalrate, 100 Angstroms per minute, of silicon nitride by phosphoric acidsupports a highly manufacturable process, and minimizes damage tounderlying active areas on the device. The invention has been describedwith reference to specific embodiments, however, it is not intended tobe limited to the illustrated embodiments, but rather it is intended tocover modifications and variations that can be made without departingfrom the spirit of the invention.

[0043] The method for fabrication of sub-micron polysilicon features hasbeen described using a specific bilayer anti-reflective coating, whichsubsequently serves as an etch mask. However, different combinations ofmaterials and processes can be used as long as the combined “n” and “k”values are developed for a thin photoresist necessary with DUV exposure,and the processes are compatible with manufacturing tolerances fordeposition, etch, and clean-up. An alternate bilayer ARC coating is asilicon nitride over silicon nitride bilayer, wherein the depositionparameters are tailored to provide the necessary “n” and “k” values.Computer simulations followed by fabrication and measurement of teststructures is the preferred approach for such development. Further, thepolysilicon structures, oxide layers, and photoresist types can bevaried according to manufacturing preferences.

What is claimed is: 1- A method for fabricating polysilicon structureson a semiconductor substrate including the steps of: a) depositing apolysilicon layer on the substrate; c) depositing on the polysiliconlayer an antireflective coating comprising a bilayer of inorganic filmshaving thicknesses, extinction coefficients, and refractive indiceswhich allow less than 1% reflection radiation at a wavelength of about193 nm; c) depositing a photoresist over the antireflective coating; d)exposing selected portions of the resist to radiation of about 193 nmwavelength; e) developing the resist to create a resist mask definingand protecting portions of said anti-reflective coating; f) patterningsaid bilayer antireflective coating to form a hard mask, therebyexposing selected areas of said polysilicon; g) removing thephotoresist; h) etching to remove the exposed polysilicon, i) etching toremove the second or top layer of antireflective hard mask; and j)removing the first or bottom layer of hard mask simultaneously with thepolysilicon clean-up process. 2- A method as in claim 1 wherein saidanti-reflective coating-comprises a first layer of silicon rich nitride,and a second layer of silicon oxynitride, 3- A method as in claim 1wherein the thickness of the first layer of said antireflective coatingis between 300 and 1500 angstroms, and the second layer is in the rangeof 170 to 330 angstroms. 4- A method as in claim 1 wherein theextinction coefficient of said second layer is in the range of 0.3 to0.4. 5- A method as in claim 1 wherein the extinction coefficient ofsaid first layer of the antireflective coating is in the range of 0.77to 1.07. 6- A method as in claim 1 wherein the polysilicon thickness isin the range of 1200 to 2500 angstroms. 7- A method as in claim 1wherein said antireflective coatings are deposited by plasma enhancedchemical vapor deposition. 8- A method as in claim 1 wherein said postpolysilicon etch utilizes phosphoric acid. 9- A method as in claim 1wherein said photoresist, selected areas of said antireflective coating,and selected areas of said polysilicon are removed in-situ in the sameplasma etch chamber. 10- A method as in claim 1 wherein silicon nitrideis rapidly removed by phosphoric acid at a rate of about 100 angstromsper minute. 11- A method as in claim 1 wherein said photoresist is about2000 to 3000 angstroms thickness. 12- A method for making an inorganicantireflective coating having less than 1% reflection at 193 nmwavelength, comprising the steps of: depositing an antireflectivebilayer directly on a reflective substrate, said bilayer comprisingsilicon rich silicon nitride and silicon oxynitride. 13- The method asin claim 12 wherein said reflective substrate comprises polysilicon. 14-The method as in claim 12 wherein the step of forming an antireflectivebilayer comprises a first layer having an extinction coefficient in therange of 0.77 to 1.07, and the second layer having an extinctioncoefficient in the range of 0.3 to 0.4. 15-A method as in claim 12wherein the thickness of said first layer is greater than 300 angstroms,and the thickness of said second layer is in the range of 170 to 330angstroms. 16- A method of forming an inorganic hard mask for protectingselected areas of polysilicon during an etching to form a semiconductordevice comprising the steps of: providing a substrate with a layer ofpolysilicon; depositing layer of silicon rich silicon nitride atop thepolysilicon; depositing a layer of silicon oxynitride over the siliconnitride; depositing a layer of photoresist over the silicon nitride;exposing selected areas of said photoresist to ultraviolet radiation;developing the resist to create a resist mask; etching unmasked portionsof the silicon oxynitride and silicon nitride; removing the resist, andetching to remove the silicon oxynitride and exposed portions ofpolysilicon; and removing the silicon nitride employing the phosphoricacid clean-up of polysilicon. 17- A method as in claim 16 wherein thehard mask is an anti-reflective coating having less than 1% reflection.18- A method as in claim 16 wherein the silicon nitride layer is rapidlyetched by phosphoric acid. 19- An anti-reflective coating comprising abilayer of silicon rich silicon nitride in the range of 300 to 1500angstroms thickness, and a layer of silicon oxynitride in the range of170 to 330 angstroms thickness. 20- An antireflective coating as inclaim 19 wherein the extinction coefficient of the silicon nitride layeris in the range of 0.77 to 1.07, and the silicon oxynitride extinctioncoefficient is in the range of 0.3 to 0.4. 21- An antireflective coatingas in claim 19 wherein reflectivity is less than 1% at 193 nm wavelengthexposure.